DESCRIPTION (Taken directly from applicant's abstract) Focal segmental glomerulosclerosis is the second most common cause of kidney failure in childhood. The specter of FSGS is present when nephrotic syndrome develops in children, though most have a good prognosis. Typically, children with FSGS do no respond to therapy and ultimately progress to kidney failure. Unfortunately, we lack the ability to predict which patients with childhood nephrotic syndrome will have poor outcome. This prevents us from providing appropriate therapy to those at highest risk. The serum of patients with FSGS contains a factor that causes proteinuria. This was suggested by the rapid recurrence of proteinuria in patients who have kidney transplants for FSGS and the fact that plasmapheresis can ameliorate this proteinuria. This has been confirmed by studies showing that serum from some FSGS patients increases the protein permeability of isolated glomeruli. The presence of this factor has been associated with the development of recurrent FSGS in patients receiving kidney transplants. Our first hypothesis is that the presence of the FSGS serum permeability factor ("factor") at the initial presentation of childhood nephrotic syndrome predicts a less favorable clinical prognosis. The serum from children with nephrotic syndrome (NS) will be assayed for the factor at the initial presentation of the nephrotic syndrome. This will define the utility of factor detection in altering our approach to patient treatment. Our second hypothesis is that the presence of the factor predicts the histology of patients who receive kidney biopsies for childhood nephrotic syndrome. Serum from children who have a kidney biopsy will be assayed for the presence of the factor. Biopsies will be reviewed by a single pathologist, permitting an analysis of the relationship between histology and the factor. Our third hypothesis is that there will be a correlation between clinical characteristics and the presence of the factor in children with nephrotic syndrome. This data will be analyzed for all children who enter this study. Our fourth hypothesis is that the disappearance of the factor is associated with a more favorable clinical course. Factor positive children will be analyzed for the presence of the factor throughout their clinical course.